Current commercial aircraft include the capability to be flown either manually or using automatic control systems. For example, in many aircraft, it is typical to have an autothrottle, an autopilot, and a Flight Management Computer (FMC). Each of these systems is interrelated to the others, with a hierarchy of control levels existing between the systems, e.g., flight management computers are arranged to control both the autopilot and the autothrottle, autopilots are arranged to control the autothrottle, etc. By adding a Mode Control Panel (MCP), a wide range of flight modes become available for use by the pilot.
When landing an aircraft (either manually or using an automatic mode), it is typical to use the autothrottle to reduce the engine thrust to idle when the aircraft reaches a certain altitude. For example during an automatic landing, upon reaching 24 feet, the autothrottle will move the engine control lever to idle. The rate at which the lever is moved depends on the existing lever position, e.g., a lever position that is already close to idle will move slower than a lever position that is farther from idle. Typical throttle lever rate movements are between about -2.2 degrees per second to about -1.7 degrees per second. A negative sign refers to a resulting reduction in engine throttle setting. The vertical rate at which the aircraft actually lands on the runway (referred to as the vertical speed or sink rate at touchdown) is often influenced by current wind and weather conditions, and is quite dependent upon the reduction of throttle to idle.
During the aircraft landing maneuver, wind changes that reduce the airplane airspeed (referred to as an underspeed condition) may result in a reduction of airplane elevator effectiveness and in a high sink rate at touchdown. This may be felt as a hard bump at touchdown, which can cause discomfort to some passengers and can cause wear to the landing gear. Underspeed conditions are also associated with short landing distances.
Conversely, a wind change resulting in an airspeed increase (referred to as an overspeed condition) will result in increased lift on the wing and oppose the elevator commands to land the airplane on the runway. During overspeed conditions the airplane will float down the runway, resulting in a very soft landing but undesirably requiring excessive runway distance within which to decelerate and come to a full stop.
Thus, a need exists for an autothrottle system having the capability to guide the aircraft during landing to avoid landing too hard in order to minimize any possibility of passenger discomfort. The ideal system would further foresee and help to avoid those situations in which excessive runway distance may be required for touchdown and that will result in less runway within which to decelerate. The present invention is directed to fulfilling these needs.